yop: simplify/sanitize the decoding loop

Do not overwrite linesize set by get_buffer().

The last frame in the FATE test is not decoded anymore, since the file
is cut and a part of it is missing.

libavcodec/yop.c

@@ -36,7 +36,6 @@ typedef struct YopDecContext {
     int num_pal_colors;
     int first_color[2];
     int frame_data_length;
-    int row_pos;
 
     uint8_t *low_nibble;
     uint8_t *srcptr;
@@ -173,27 +172,12 @@ static uint8_t yop_get_next_nibble(YopDecContext *s)
     return ret;
 }
 
-/**
- * Take s->dstptr to the next macroblock in sequence.
- */
-static void yop_next_macroblock(YopDecContext *s)
-{
-    // If we are advancing to the next row of macroblocks
-    if (s->row_pos == s->frame.linesize[0] - 2) {
-        s->dstptr  += s->frame.linesize[0];
-        s->row_pos =  0;
-    }else {
-        s->row_pos += 2;
-    }
-    s->dstptr += 2;
-}
-
 static int yop_decode_frame(AVCodecContext *avctx, void *data, int *got_frame,
                             AVPacket *avpkt)
 {
     YopDecContext *s = avctx->priv_data;
     int tag, firstcolor, is_odd_frame;
-    int ret, i;
+    int ret, i, x, y;
     uint32_t *palette;
 
     if (s->frame.data[0])
@@ -205,12 +189,9 @@ static int yop_decode_frame(AVCodecContext *avctx, void *data, int *got_frame,
         return ret;
     }
 
-    s->frame.linesize[0] = avctx->width;
-
     s->dstbuf     = s->frame.data[0];
     s->dstptr     = s->frame.data[0];
     s->srcptr     = avpkt->data + 4;
-    s->row_pos    = 0;
     s->low_nibble = NULL;
 
     is_odd_frame = avpkt->data[0];
@@ -224,9 +205,12 @@ static int yop_decode_frame(AVCodecContext *avctx, void *data, int *got_frame,
 
     s->frame.palette_has_changed = 1;
 
-    while (s->dstptr - s->dstbuf <
+    for (y = 0; y < avctx->height; y += 2) {
-           avctx->width * avctx->height &&
+        for (x = 0; x < avctx->width; x += 2) {
-           s->srcptr - avpkt->data < avpkt->size) {
+            if (s->srcptr - avpkt->data >= avpkt->size) {
+                av_log(avctx, AV_LOG_ERROR, "Packet too small.\n");
+                return AVERROR_INVALIDDATA;
+            }
 
             tag = yop_get_next_nibble(s);
 
@@ -240,7 +224,9 @@ static int yop_decode_frame(AVCodecContext *avctx, void *data, int *got_frame,
                     return ret;
                 }
             }
-        yop_next_macroblock(s);
+            s->dstptr += 2;
+        }
+        s->dstptr += 2*s->frame.linesize[0] - x;
     }
 
     *got_frame = 1;

tests/ref/fate/yop

@@ -5,4 +5,3 @@
 0,          3,          3,        1,   302760, 0xc34b20bd
 0,          4,          4,        1,   302760, 0x461d29a1
 0,          5,          5,        1,   302760, 0x45abca02
-0,          6,          6,        1,   302760, 0xb05448b9